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  #1  
Old 03-18-2023, 08:00 PM
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Plummit Plummit is offline
 
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Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc
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  #2  
Old 03-18-2023, 08:16 PM
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Tankerpilot75 Tankerpilot75 is offline
 
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Location: Oklahoma City, OK
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Quote:
Originally Posted by Plummit View Post
Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc
If you have the light weight FlyTech starter I suspect the starter solenoid on the starter has failed. You can either buy a new starter for megabucks or replace the existing one with a Ford starter solenoid for around $55. There are several threads on VAF that discuss this issue. Itís not a hard job. Good luck.
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  #3  
Old 03-18-2023, 08:19 PM
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PaulvS PaulvS is offline
 
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Sounds like a short somewhere, I think I'd do a bit more investigating before putting in a bigger fuse!
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  #4  
Old 03-18-2023, 08:45 PM
blaplante blaplante is offline
 
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Location: Southern California
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So the shunt is in-line when you hit the starter?
Very unusual.
And skytec says https://skytec.aero/aircraft-starter...ked-questions/
can be 190 amps inrush current ... so the typical 100Amp shunt isn't going to live forever if so.
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  #5  
Old 03-18-2023, 08:49 PM
Chipper G Chipper G is offline
 
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Should not have any fuse in the starter main cable. The sky Tec can pull 150 amps easy. Ask me how I know
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  #6  
Old 03-18-2023, 08:59 PM
Mich48041 Mich48041 is offline
 
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A shunt has a certain amount of resistance. Any resistance in the starter circuit is going to slow the starter down.
Remove that shunt.
If you still want to measure current, use a hall effect sensor which is usually placed around a positive wire, not the grounded cable.
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  #7  
Old 03-18-2023, 09:59 PM
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Plummit Plummit is offline
 
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Quote:
Originally Posted by blaplante View Post
So the shunt is in-line when you hit the starter?
Very unusual.
The shunt is on the ground wire before the forest of tabs so yes, all of the current for the starter goes through the shunt. It's worked flawlessly for over 1000 hobbs hours. I find it strange that the the shunt is on the ground too. The instructions say to put it on the positive cable.

I'm running a 70 amp PP alternator so a 60 amp shunt seems too small anyway.

-Marc
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Last edited by Plummit : 03-18-2023 at 10:09 PM.
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  #8  
Old 03-18-2023, 10:24 PM
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PaulvS PaulvS is offline
 
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"It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero."

I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?

There's a couple of options for correctly locating the current sensing shunt, depending on whether the aim is to measure what's going into the battery, or, what is the equipment load (excluding the starter). If the latter then a 60A shunt is still appropriate.
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Building RV-6A #22320 O-320 FP. Airframe structure 95% complete, now working on integration, plumbing and wiring.
Flying my Aeroprakt A-22 STOL and the aero club's RV-9A while I build
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  #9  
Old 03-18-2023, 11:06 PM
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Plummit Plummit is offline
 
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Quote:
Originally Posted by PaulvS View Post
I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?
It was probably back feeding through one of the light or device circuits as they are not LED.

Quote:
Originally Posted by paulvS
There's a couple of options for correctly locating the current sensing shunt, depending on whether the aim is to measure what's going into the battery, or, what is the equipment load (excluding the starter). If the latter then a 60A shunt is still appropriate.
Yes I agree. The problem I have now is relocating the shunt to the engine side of the firewall so I can put it on a positive cable after the starter. There are 2 shunts there already.

I have a PP alternator and a B&C back-up alternator. I don't know what the failed shunt was measuring. I have a 900X panel with an informative EIS.

-Marc
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  #10  
Old 03-19-2023, 12:49 AM
gasman gasman is offline
 
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Quote:
Originally Posted by Plummit View Post
Yup, pulled the plane out for breakfast run this morning, hit the starter and it did it's usual thing where it couldn't get past the compression stroke of the first cyl. I released the starter then hit it again. Same thing so I released it and hit it one more time - Everything, including the fuel pump shut off. Dead, nada nothing.

Talked to 2 A&P friends and the builder and they all said "Dead Battery".

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home! I'll upgrade the 60 to a 100 amp when I can get one next week.

-Marc
This #2 cable was grounded first to the firewall brace on the engine side before it went to the panel... RIGHT?

If the shunt was mounted to the cabin side of the firewall, was the cable also connected (grounded) to the firewall? If it is, then the shunt is not the starting problem unless you don't have a ground cable from the firewall or battery - to the motor. And if that is the case, your starter has been grounding through any metal between the airframe and the motor....not good! I don't think this is just a case of just replacing the shunt, you need to confirm the ground path from the battery to the starter.

Look for ground cables going from the engine to the firewall... There should be at least one..... and if the builder was smart, there will be two.
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Last edited by gasman : 03-19-2023 at 01:27 AM.
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